To support large numbers of (roaming) subscribers—e.g. mobile subscribers or nomadic, i.e. temporarily fixed subscribers—a number of telecommunications networks such as public land mobile networks and/or stationary (fixed line) networks have been established and are operative. Telecommunications networks such as public land mobile networks (PLMNs) may communicatively connect to other mobile telecommunications networks and/or to fixed, land-wired public switched telephone networks (PSTNs) and/or to data networks such as the internet (or IP-networks, internet protocol networks). User equipments (e.g. a mobile or nomadic terminal) are typically assigned to a home telecommunications network, such as a home public land mobile network. In a roaming scenario, the user equipment is typically not located in reach of the home telecommunications network (or home public land mobile network) but is connected to a visited telecommunications network (or visited public land mobile network). Most modern telecommunications network (such as public land mobile networks or also data networks) are packet-switched networks, in which data are routed in autonomous units called data packets. Packet-switched networks are characterized by high speed, low latency, and high network resource utilization efficiency. Two examples of packet-switched technologies are the General Packet Radio Service (GPRS) of the Global System for Mobile communications (GSM), and the Evolved Packet System (EPS), also commercially known as the Long Term Evolution (LTE), of the Universal Mobile Telecommunications System (UMTS), wireless local area network (WLAN). The Internet Protocol (IP) is a widely-deployed protocol for data communications in packet-switched networks. The IP Multimedia System (IMS) is an industry standard framework for delivering voice and multimedia communications over IP networks. Communications between nodes within an IMS network utilize the Session Initiation Protocol (SIP). SIP is a signaling protocol for Internet conferencing, telephony, presence, events notification, instant messaging, and the like. SIP signaling uses a long-term stable identifier, the SIP Universal Resource Indicator (URI). SIP signaling data packets in an IMS network are processed by network node of the telecommunications network, usually also called SIP servers or proxies. Together, these network nodes provide a call session control function (CSCF). Different types of such CSCF network nodes perform specific functions: A proxy call session control function (P-CSCF) is a SIP proxy that is the first point of contact for a user equipment (or an IMS terminal). Typically, the proxy call session control function that a user equipment uses is located in the visited telecommunications network in case that local breakout is used. In case local breakout is not used, the proxy call session control function a user equipment normally uses is located in the home telecommunications network (i.e. the proxy call session control function is part of or is assigned to the home telecommunications network).
Also IMS-enabled user equipments need to be able to place emergency calls even when located in (and connected to) a visited telecommunications network. Non-UE-detected emergency calls (i.e. emergency calls that are placed (or initiated) by the user equipment which are emergency calls in the network environment of the user equipment (i.e. the visited telecommunications network) that the user equipment is unaware of, e.g. due to the use of an emergency call telephone number (valid in the visited telecommunications network) that is not maintained in a list within the user equipment) are relatively easy to handle in case of the use of local breakout: In this case, the local proxy call session control function (i.e. the proxy call session control function of the visited telecommunications network) has a list of the local emergency numbers (of the visited telecommunications network or valid with in the respective context of the user equipment in the visited telecommunications network), and is hence able to detect the call type (i.e. the emergency type of an emergency call) and apply the needed procedures to guarantee successful handling of the requested emergency service.
In scenarios where no local breakout is to be used, and each call is routed to (a respective proxy call session control function within) the home telecommunications network, there is only the possibility to maintain a (normally huge) list of local emergency numbers (that are applicable or valid within the different telecommunications networks of all the possible roaming partners of the home telecommunications network) with a mapping to the related uniform resource names of the emergency call telephone numbers (also called sos-uniform resource names) in the local network node or the home telecommunications network (typically a proxy call session control function of the home telecommunications network). The maintenance of such a database for all roaming partners worldwide requires a considerable effort and—hence—is very costly.